1. Technical Field
The present invention relates to a projector, and, more particularly to a projector that can display a visible image and an infrared image on one screen.
2. Related Art
A projector that displays an optical image, which is image information formed on a light modulation element, on a screen in enlargement is highly consistent with digital apparatuses such as a computer. Therefore, the projector is widely used mainly for presentation, information provision, and video entertainment. Since a major function of a general display apparatus including the projector is to communicate some meaningful information to people, image information is usually displayed as a visible image that people can visually recognize. Therefore, a general projector selects visible light from light radiated from a light source and uses the visible light as illumination light to perform image display.
It is said that, when meaningful information is displayed for presentation or information provision, there is an information amount suitable for display in terms of visibility, readability, information acceptability, and the like. This is because, when an excessively large amount of information is displayed at a time, people cannot accurately grasp the information, for example, it is difficult to find necessary information in busy streets in large cities where a large number of signboards are hanged out, but also tend to neglect to pay attention to individual pieces of information.
For presentation and video entertainment, it is significant to ensure security of information to be displayed. As a growing social problem, presentation information and video information are photographed and diverted without permission of copyright holders.
To cope with such a problem, it is considered to control displayed information (images and videos) using infrared light that people cannot visually recognize. People cannot recognize an image displayed by the infrared light (an infrared image). However, since an imaging device represented by a CCD and a CMOS sensor has sensitivity covering an infrared region, people can visually recognize display content using the imaging device. For example, in Yoshinari Shirai and Mitsunori Matsushita, “Proposal of Method of Providing Information to an Actual Environment using an IR projector”, Information Processing Technology Letters (FIT2003), Vol. 2, pp. 287 to 289, 2003 (see FIG. 1), a method of using a normal projector for visible light, which displays a color visible image, and a projector for infrared light, which displays an image with infrared light, in combination to thereby control information (images and videos) to be displayed is proposed. Specifically, information desired to be shown to an unspecified large number of people to communicate presence and an outline of information is displayed as a visible image and information desired to be shown to people who require more detailed information is displayed as an invisible infrared image. People see the invisible infrared image using an imaging device such as a digital camera.
From the viewpoint of ensuring of security of information, for example, U.S. Pat. No. 6,018,374 proposes a method of projecting, using a projector for infrared light, an infrared image over an image projected on a screen, and causing, when the image projected on the screen is imaged by an imaging device such as a digital video or a digital camera, the infrared image to be taken into the imaged image to thereby inform a copyright holder that the photographing is an illegal act and prevent secret photography and the like.
As described in the documents described above, when the techniques are adopted, it is possible to control displayed information (images and videos) by displaying an invisible infrared image over a visible image displayed on a screen. Therefore, the techniques are useful for prevention of illegal acts such as secret photography. However, in addition to a projector that projects and displays the original information, a second projector that projects the infrared image is necessary. Taking into account the fact that there is an increasing number of portable projectors according to a reduction in size of projectors, portability is impeded when the two projectors are used. Moreover, in using the two projectors, it is necessary to accurately adjust projection and display positions of the projectors to each other. Every time each of the two projectors is moved, work for accurately adjusting projection and display positions of the projectors to each other is necessary. In particular, since the infrared image is an invisible image, the work cannot be performed visually. Therefore, installability and usability are extremely deteriorated.